1. Field of the Invention
This invention relates to particle acceleration devices and methods thereof. More particularly, the invention relates to particle acceleration devices and methods used for measuring properties of subterranean formations such as in borehole logging or wellbore applications.
2. Background of the Invention
Nuclear borehole logging measurements typically employ one or more unstable radio-chemical isotopes such as 137Cs or AmBe to generate fixed-energy gamma or neutron radiation (logging sources). Due to the requirements of the oil industry, such sources are of extremely high intensity and radio-activity, often exceeding 2 Ci for 137Cs and 20 Ci for AmBe. As such, their deployment in oilfields worldwide is strictly controlled and regulated. The use of such sources forces the well-logging industry to manage great safety and security risks.
Alternative, “source-less” methods exist such as X-ray tubes, betatrons and minitrons (see e.g., U.S. Pat. Nos. 5,122,662 and 5,293,410 by F. Chen et al.). X-ray tubes are essentially electro-static accelerators and as such they are limited to energies of a few 100 KeV that can be reached with DC electric fields. Betatrons are in principle capable to reach very high energies however it remains a challenge to do so in the confined space of a logging tool. Minitrons are powerful, extremely compact neutron sources, however reaching further increases in output and lifetime remains extremely challenging. Linear accelerators can be utilized to accelerate electrons onto a radiator target to produce X-rays or to accelerate protons or other nuclei onto nuclear targets (e.g., Be, Li) to produce neutrons. Linear acceleration schemes based on traditional RF acceleration from a pillbox type microwave cavity (normally conducting pill box cavity) are notoriously difficult to scale for borehole applications, given the excessive power consumption, tool length and tool weight. As such they have never been employed in the oilfield.
An acceleration method is disclosed that relates to photonic band gap cavities (PBG cavity). A suitably designed resonator based on a PBG structure confines only the desired oscillating modes of electromagnetic fields, such as those required for particle acceleration. This property of a PBG cavity is well described in the scientific literature, including, for example J. D. Joannopoulos, R. D. Meade, and J. N. Winn, Photonic Crystals: Molding the Flow of Light (Princeton, N.J.: Princeton University Press, 1995).
With a PRG resonator operating at microwave frequencies in the GHz region, the RF power coupled externally via—e.g.—a coaxial loop or a wave-guide, can be concentrated in a very small volume providing a localized accelerating gradient. Mode selection inside the cavity ensures that only the wanted acceleration modes are present. This allows for an efficient use of RF power in an ideal compact geometry where wall losses are greatly reduced. The underlying principle of PBG cavity is universal and as such PBG cavities can operate in a broad range of frequencies.
A PBG-based electro-magnetic resonator (a cavity) consists of a symmetrical arrangement of plates and rods. An inverse structure with a symmetrical arrangement of cylindrical holes bored into a solid template may also be used. In either case the periodic structure is designed in such a way that the propagation of electro-magnetic waves in certain TE and/or TM modes in a given frequency range (the band-gap) is effectively forbidden. This feature depends principally on the boundary conditions and the geometry of the cavity.
A suitable PBG cavity would consist of symmetric plate-rod structure. Such a structure would also contain one or more introduced defects such as a missing or partially withdrawn rod. The volume around the defect is open to the electromagnetic mode whose propagation is elsewhere blocked by the band gap. In other words, the modes in the band gap are confined to the rod structure only and are by their very nature discrete. By introducing a defect while still preserving the symmetry properties of the resonator we have access to the confined, mode-selected fields that would otherwise be confined inside the rods. These fields effectively are those of a resonant cavity. Similarly, when the cavity consists of holes: the electro-magnetic modes may be confined to the holes.
U.S. Pat. No. 6,801,107B2 by Temkin et al. describes a PBG cavity that is suitable for frequency-filtering in the microwave regime. In particular, the Temkin device relates to vacuum electron devices that comprises a Photonic Band Cap (PBG) structure (or cavity) capable of overmoded operation, as well single mode operation. One distinct advantage of PBG cavities used for particle acceleration relative to prior art is that practically all undesired higher-order electromagnetic modes are not confined by the defect structure and therefore leak away with minimal effect on the electrons or ions in the beam.